1. Field of the Invention
The present invention relates to a loop antenna, and particularly, to a dual-band loop antenna to be built in a mobile telephone, which generates a half-wavelength mode and a full-wavelength mode of a radiating metallic loop thereof, and an additional half-wavelength mode by coupling between a radiating metallic plate thereof and the radiating metallic loop.
2. Description of the Related Art
Following fast development of wireless communication technology, a variety of wireless communication products have been developed and have appeared on the market. In wireless communication products, the antenna plays an important role. Under the tendency toward the design having light, thin, short and small characteristics, the height of the antenna of a wireless communication apparatus determines the value of the product. Modern wireless communication products commonly adopt internal planar antennas or external monopole antennas.
The internal planar antennas of conventional dual-band mobile telephones commonly have a height about 7-10 mm. A conventional dual-band planar inverted-F antenna is known comprising an internal or concealed cell phone antenna having a radiating metallic plate and a system ground plane for GSM (Global System for Mobile Communications) and DCS (Digital Cellular System) dual-band application. Because of the concealed cell phone antenna, the cell phone has a height over 10 mm.
When we have the aforesaid concealed cell phone antenna be used in a thin-thickness cell phone (of thickness about 10 mm or thinner), we will encounter a problem of over-sized antenna and will be unable to satisfy the requirements of having cell phones be realized in thin-thickness.
To avoid this problem, manufacturers commonly adopt a monopole antenna design in which the monopole antenna protrudes over the ground plane and is less affected by the ground plane, allowing the thickness of the cell phone to be minimized.
However, either a planar antenna or monopole antenna is used, subject to the limited space inside the cell phone, the antenna can cover only the bandwidth of the GMS band. When the environment of use is changed, for example, when the user holds the cell phone with the hand or keeps the cell phone close to his (her) head, the large dielectric coefficient of hand or head will cause a frequency shift, resulting in a decrease in cell phone radiation efficiency.